Methanoic acid mixes with water in all proportions and…
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Methanoic acid mixes with water in all proportions and has about the same boiling point as water. Which of the following methods would you adopt to obtain pure water from a mixture of sand, water and methanoic acid?
A. Fractional distillation
B. Filtration followed by distillation
C. Neutralization with sodium hydroxide followed by distillation
D. Neutralization with sodium hydroxide followed by filtration
Be careful with this question…
If methanoic acid is completely soluble in water and boils at almost the same temperature, then fractional distillation will not be able to separate them, this is because fractional distillation takes advantage of differences in boiling points of individual components of a mixture in order to separate them.
Now, if we neutralize the mixture with sodium hydroxide, the products will be, sodium methanoate, water, and sand. At this point please be careful, sodium methanoate is a white powdery solid which is quite soluble in water. Therefore, filteration at this point will not obtain pure water but only obtain sand from the mixture and the water remaining will contain dissolved sodium methanoate.
But applying direct distillation to sand, water, and dissolved sodium methanoate, will separate water from the mixture, leaving behind solid sodium methanoate and sand. Remember, our interest is to obtain pure water from the initial mixture.
Now for the right answer to the above question:
- Option A is incorrect. fractional distillation will result in the vaporization of methanoic acid and water at the same time.
- Option B is also incorrect. our interest is to obtain pure water from the mixture and not sand, so, filtration is not needed here. Direct distillation also is not reasonable.
- C is correct. Neutralization of the mixture with sodium hydroxide will yield dissolved sodium methanoate, water, and sand. And then distillation of this will extract water, leaving behind solid sodium methanoate and sand.
- D is not the correct answer. Filtration will not give pure water, as the sodium methanoate will be completely dissolved in the water.
You may please note these/this:
- Remember, our interest is to obtain pure water from the initial mixture.
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/ culled from 2018 JAMB-UTME Chemistry question 11 /